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Date: Tue, 22 May 2012 23:31:43 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Gilad Ben-Yossef <gilad@benyossef.com>,
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Subject: Re: [RFC][PATCH 00/32] Nohz cpusets v2 (adaptive tickless kernel)
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On Tue, 27 Mar 2012, Peter Zijlstra wrote:

> On Tue, 2012-03-27 at 17:02 +0200, Gilad Ben-Yossef wrote:
> > 
> > In my case, I also had to disable the clocksource watchdog, but only
> > because TSC is not stable on my VM.
> > This is really not a nohz/cpuset problem. 
> 
> No but that thing is annoying, I ran afoul of it too the other day.
> 
> Thomas, would you object to a means of turning that thing off? And if
> not, do you have a preference as to what particular means
> (sysctl/sysfs/etc..) ?

We have the commandline option "tsc=reliable" already. That disables
the stupid watchdog. Handle with care.

To take it a level further, use the patch below :)

Thanks,

	tglx

------------>
x86: Add tsc=perfect option which enforces sched_clock_stable
    
The sched_clock dance with updating local clocks can be avoided even
if the CPU does not have all the magic features.
    
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 590900c..dc8ecc3 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -109,6 +109,10 @@ static int __init tsc_setup(char *str)
 {
 	if (!strcmp(str, "reliable"))
 		tsc_clocksource_reliable = 1;
+	if (!strcmp(str, "perfect")) {
+		tsc_clocksource_reliable = 1;
+		sched_clock_stable = 1;
+	}
 	if (!strncmp(str, "noirqtime", 9))
 		no_sched_irq_time = 1;
 	return 1;